There is conventionally known a turbocharger that includes: a turbine wheel attached to one end of a shaft member and rotated by exhaust gas energy; and a compressor wheel coaxially attached to the other end of the shaft member and adapted to compress air and supply the compressed air to a combustion chamber of an internal combustion engine.
Patent Document 1 discloses a turbocharger of the above-mentioned type, which further includes an electric motor disposed on the shaft member between the turbine wheel and the compressor wheel such that, even in the case where adequate supercharging cannot be performed due to insufficient rotation speed of the turbine wheel during a low speed state of the internal combustion engine at e.g. a vehicle start, the electric motor can be driven to increase the rotation speed of the compressor wheel, raise the boost pressure of the turbocharger and thereby obtain required power performance from the early stage of the vehicle start.
In Patent Document 1, the electric motor is required to achieve high rotation and high output for the purpose of imparting desired torque to the shaft member. However, the high-rotation, high-output electric motor becomes large in size. The use of such a large-size motor can lead to an increase in the overall size of the turbocharger.
Furthermore, a large amount of heat is generated from a stator coil (as a stator) of the high-rotation, high-output electric motor. Due to such heat generation, a magnet (as a rotor) of the electric motor may be deteriorated by being heated to a high temperature. This can cause a decrease in the driving efficiency of the electric motor by deterioration of magnetic force.